RTL

1Henry HexmoorHenry Hexmoor 1

Chapter 7Chapter 7Henry HexmoorHenry Hexmoor

Registers and RTLRegisters and RTL

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REGISTER TRANSFER AND MICROOPERATIONS

• Register Transfer Language

• Register Transfer

• Bus and Memory Transfers

• Arithmetic Microoperations

• Logic Microoperations

• Shift Microoperations

• Arithmetic Logic Shift Unit

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SIMPLE DIGITAL SYSTEMS

• Combinational and sequential circuits can be used to create simple digital systems.

• These are the low-level building blocks of a digital computer.

• Simple digital systems are frequently characterized in terms of– the registers they contain, and

– the operations that they perform.

• Typically,– What operations are performed on the data in the registers

– What information is passed between registers

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MICROOPERATIONS (1)Register Transfer Language

• The operations on the data in registers are called microoperations.

• The functions built into registers are examples of microoperations

– Shift

– Load

– Clear

– Increment

– …

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MICROOPERATION (2)

An elementary operation performed (during one clock pulse), on the information stored in one or more registers

R f(R, R)

f: shift, load, clear, increment, add, subtract, complement,and, or, xor, …

ALU(f)

Registers(R) 1 clock cycle

Register Transfer Language

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ORGANIZATION OF A DIGITAL SYSTEM

- Set of registers and their functions

- Microoperations set

Set of allowable microoperations provided by the organization of the computer

- Control signals that initiate the sequence of microoperations (to perform the functions)

• Definition of the (internal) organization of a computer

Register Transfer Language

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REGISTER TRANSFER LEVELRegister Transfer Language

• Viewing a computer, or any digital system, in this way is called the register transfer level

• This is because we’re focusing on– The system’s registers

– The data transformations in them, and

– The data transfers between them.

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REGISTER TRANSFER LANGUAGERegister Transfer Language

• Rather than specifying a digital system in words, a specific notation is used, register transfer language

• For any function of the computer, the register transfer language can be used to describe the (sequence of) microoperations

• Register transfer language– A symbolic language

– A convenient tool for describing the internal organization of digital computers

– Can also be used to facilitate the design process of digital systems.

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DESIGNATION OF REGISTERSRegister Transfer Language

• Registers are designated by capital letters, sometimes followed by numbers (e.g., A, R13, IR)

• Often the names indicate function:– MAR - memory address register

– PC - program counter

– IR - instruction register

• Registers and their contents can be viewed and represented in various ways

– A register can be viewed as a single entity:

– Registers may also be represented showing the bits of data they contain

MAR

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DESIGNATION OF REGISTERSRegister Transfer Language

R1 Register

Numbering of bits

Showing individual bits

Subfields

PC(H) PC(L)15 8 7 0

- a register - portion of a register - a bit of a register

• Common ways of drawing the block diagram of a register

7 6 5 4 3 2 1 0

R215 0

• Designation of a register

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REGISTER TRANSFERRegister Transfer

• Copying the contents of one register to another is a register transfer

• A register transfer is indicated as

R2 R1

– In this case the contents of register R1 are copied (loaded) into register R2

– A simultaneous transfer of all bits from the source R1 to the destination register R2, during one clock pulse

– Note that this is a non-destructive; i.e. the contents of R1 are not altered by copying (loading) them to R2

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REGISTER TRANSFERRegister Transfer

• A register transfer such as

R3 R5

Implies that the digital system has

– the data lines from the source register (R5) to the destination register (R3)

– Parallel load in the destination register (R3)

– Control lines to perform the action

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CONTROL FUNCTIONSRegister Transfer

• Often actions need to only occur if a certain condition is true

• This is similar to an “if” statement in a programming language

• In digital systems, this is often done via a control signal, called a control function

– If the signal is 1, the action takes place

• This is represented as:

P: R2 R1

Which means “if P = 1, then load the contents of register R1 into register R2”, i.e., if (P = 1) then (R2 R1)

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HARDWARE IMPLEMENTATION OF CONTROLLED TRANSFERS

Implementation of controlled transfer

P: R2 R1

Block diagram

Timing diagram

Clock

Register Transfer

Transfer occurs here

R2

R1

Control Circuit

LoadP

n

Clock

Load

t t+1

• The same clock controls the circuits that generate the control function and the destination register• Registers are assumed to use positive-edge-triggered flip-flops

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SIMULTANEOUS OPERATIONSRegister Transfer

• If two or more operations are to occur simultaneously, they are separated with commas

P: R3 R5, MAR IR

• Here, if the control function P = 1, load the contents of R5 into R3, and at the same time (clock), load the contents of register IR into register MAR

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BASIC SYMBOLS FOR REGISTER TRANSFERS

Capital letters Denotes a register MAR, R2 & numerals Parentheses () Denotes a part of a register R2(0-7), R2(L)

Arrow Denotes transfer of information R2 R1

Colon : Denotes termination of control function P:Comma , Separates two micro-operations A B, B

A

Symbols Description Examples

Register Transfer

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CONNECTING REGISTRSRegister Transfer

• In a digital system with many registers, it is impractical to have data and control lines to directly allow each register to be loaded with the contents of every possible other registers

• To completely connect n registers n(n-1) lines

• O(n2) cost– This is not a realistic approach to use in a large digital system

• Instead, take a different approach

• Have one centralized set of circuits for data transfer – the bus

• Have control circuits to select which register is the source, and which is the destination

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BUS AND BUS TRANSFER

Bus is a path(of a group of wires) over which information is transferred, from any of several sources to any of several destinations.

From a register to bus: BUS R

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4Register A Register B Register C Register D

B C D1 1 1

4 x1MUX

B C D2 2 2

4 x1MUX

B C D3 3 3

4 x1MUX

B C D4 4 4

4 x1MUX

4-line bus

x

yselect

0 0 0 0

Register A Register B Register C Register D

Bus lines

Bus and Memory Transfers

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TRANSFER FROM BUS TO A DESTINATION REGISTER

Three-State Bus Buffers

Bus line with three-state buffers

Reg. R0 Reg. R1 Reg. R2 Reg. R3

Bus lines

2 x 4Decoder

Load

D0 D1 D2 D3z

wSelect E (enable)

Output Y=A if C=1High-impedence if C=0

Normal input A

Control input C

Select

Enable

0123

S0S1

A0B0C0D0

Bus line for bit 0

Bus and Memory Transfers

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BUS TRANSFER IN RTLBus and Memory Transfers

• Depending on whether the bus is to be mentioned explicitly or not, register transfer can be indicated as either

or

• In the former case the bus is implicit, but in the latter, it is explicitly indicated

R2 R1

BUS R1, R2 BUS

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MEMORY (RAM)Bus and Memory Transfers

• Memory (RAM) can be thought as a sequential circuits containing some number of registers

• These registers hold the words of memory

• Each of the r registers is indicated by an address

• These addresses range from 0 to r-1

• Each register (word) can hold n bits of data

• Assume the RAM contains r = 2k words. It needs the following

– n data input lines

– n data output lines

– k address lines

– A Read control line

– A Write control line

data input lines

data output lines

n

n

k

address lines

Read

Write

RAMunit

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MEMORY TRANSFERBus and Memory Transfers

• Collectively, the memory is viewed at the register level as a device, M.

• Since it contains multiple locations, we must specify which address in memory we will be using

• This is done by indexing memory references

• Memory is usually accessed in computer systems by putting the desired address in a special register, the Memory Address Register (MAR, or AR)

• When memory is accessed, the contents of the MAR get sent to the memory unit’s address lines

AR Memoryunit

Read

Write

Data inData out

M

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MEMORY READBus and Memory Transfers

• To read a value from a location in memory and load it into a register, the register transfer language notation looks like this:

• This causes the following to occur– The contents of the MAR get sent to the memory address lines

– A Read (= 1) gets sent to the memory unit

– The contents of the specified address are put on the memory’s output data lines

– These get sent over the bus to be loaded into register R1

R1 M[MAR]

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MEMORY WRITEBus and Memory Transfers

• To write a value from a register to a location in memory looks like this in register transfer language:

• This causes the following to occur– The contents of the MAR get sent to the memory address lines

– A Write (= 1) gets sent to the memory unit

– The values in register R1 get sent over the bus to the data input lines of the memory

– The values get loaded into the specified address in the memory

M[MAR] R1

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SUMMARY OF R. TRANSFER MICROOPERATIONSBus and Memory Transfers

A B Transfer content of reg. B into reg. A

AR DR(AD) Transfer content of AD portion of reg. DR into reg. AR

A constant Transfer a binary constant into reg. A

ABUS R1, Transfer content of R1 into bus A and, at the same time,

R2 ABUS transfer content of bus A into R2 AR Address registerDR Data registerM[R] Memory word specified by reg. RM Equivalent to M[AR]

DR M Memory read operation: transfers content of memory word specified by AR into DR

M DR Memory write operation: transfers content of DR into memory word specified by AR

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MICROOPERATIONS

• Computer system microoperations are of four types:

- Register transfer microoperations

- Arithmetic microoperations

- Logic microoperations

- Shift microoperations

Arithmetic Microoperations

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ARITHMETIC MICROOPERATIONS

Summary of Typical Arithmetic Micro-Operations


R3 R1 + R2 Contents of R1 plus R2 transferred to R3

R3 R1 - R2 Contents of R1 minus R2 transferred to R3

R2 R2’ Complement the contents of R2

R2 R2’+ 1 2's complement the contents of R2 (negate)

R3 R1 + R2’+ 1 subtraction

R1 R1 + 1 Increment

R1 R1 - 1 Decrement

• The basic arithmetic microoperations are– Addition– Subtraction– Increment– Decrement

• The additional arithmetic microoperations are– Add with carry– Subtract with borrow– Transfer/Load– etc. …

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BINARY ADDER / SUBTRACTOR / INCREMENTER

FA

B0 A0

S0

C0FA

B1 A1

S1

C1FA

B2 A2

S2

C2FA

B3 A3

S3

C3

C4

Binary Adder-Subtractor

FA

B0 A0

S0

C0C1FA

B1 A1

S1

C2FA

B2 A2

S2

C3FA

B3 A3

S3C4

M

Binary Incrementer

HAx y

C S

A0 1

S0

HAx y

C S

A1

S1

HAx y

C S

A2

S2

HAx y

C S

A3

S3C4

Binary Adder


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ARITHMETIC CIRCUIT

S1S00123

4x1MUX

X0

Y0

C0

C1

D0FA

S1S00123

4x1MUX

X1

Y1

C1

C2

D1FA

S1S00123

4x1MUX

X2

Y2

C2

C3

D2FA

S1S00123

4x1MUX

X3

Y3

C3

C4

D3FA

Cout

A0

B0

A1

B1

A2

B2

A3

B3

0 1

S0S1Cin

S1 S0 Cin Y Output Microoperation0 0 0 B D = A + B Add0 0 1 B D = A + B + 1 Add with carry0 1 0 B’ D = A + B’ Subtract with borrow0 1 1 B’ D = A + B’+ 1 Subtract1 0 0 0 D = A Transfer A 1 0 1 0 D = A + 1 Increment A1 1 0 1 D = A - 1 Decrement A1 1 1 1 D = A Transfer A


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LOGIC MICROOPERATIONSLogic Microoperations

• Specify binary operations on the strings of bits in registers– Logic microoperations are bit-wise operations, i.e., they work on the

individual bits of data

– useful for bit manipulations on binary data

– useful for making logical decisions based on the bit value

• There are, in principle, 16 different logic functions that can be defined over two binary input variables

• However, most systems only implement four of these– AND (), OR (), XOR (), Complement/NOT

• The others can be created from combination of these

0 0 0 0 0 … 1 1 10 1 0 0 0 … 1 1 11 0 0 0 1 … 0 1 11 1 0 1 0 … 1 0 1

A B F0 F1 F2 … F13 F14 F15

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LIST OF LOGIC MICROOPERATIONS• List of Logic Microoperations - 16 different logic operations with 2 binary vars. - n binary vars → functions2 2 n

• Truth tables for 16 functions of 2 variables and the corresponding 16 logic micro-operations

BooleanFunction

Micro-Operations

Namex 0 0 1 1y 0 1 0 1

Logic Microoperations

0 0 0 0 F0 = 0 F 0 Clear0 0 0 1 F1 = xy F A B AND0 0 1 0 F2 = xy' F A B’0 0 1 1 F3 = x F A Transfer A0 1 0 0 F4 = x'y F A’ B0 1 0 1 F5 = y F B Transfer B0 1 1 0 F6 = x y F A B Exclusive-OR0 1 1 1 F7 = x + y F A B OR1 0 0 0 F8 = (x + y)' F A B)’ NOR1 0 0 1 F9 = (x y)' F (A B)’ Exclusive-NOR1 0 1 0 F10 = y' F B’ Complement B1 0 1 1 F11 = x + y' F A B1 1 0 0 F12 = x' F A’ Complement A1 1 0 1 F13 = x' + y F A’ B1 1 1 0 F14 = (xy)' F (A B)’ NAND1 1 1 1 F15 = 1 F all 1's Set to all 1's

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HARDWARE IMPLEMENTATION OF LOGIC MICROOPERATIONS

0 0 F = A B AND0 1 F = AB OR1 0 F = A B XOR1 1 F = A’ Complement

S1 S0 Output -operation

Function table

Logic Microoperations

B

A

S

S

F

1

0

i

i

i0

1

2

3

4 X 1MUX

Select

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APPLICATIONS OF LOGIC MICROOPERATIONSLogic Microoperations

• Logic microoperations can be used to manipulate individual bits or a portions of a word in a register

• Consider the data in a register A. In another register, B, is bit data that will be used to modify the contents of A

– Selective-set A A + B

– Selective-complement A A B

– Selective-clear A A • B’

– Mask (Delete) A A • B

– Clear A A B

– Insert A (A • B) + C

– Compare A A B

– . . .

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SELECTIVE SETLogic Microoperations

• In a selective set operation, the bit pattern in B is used to set certain bits in A

1 1 0 0 At

1 0 1 0 B

1 1 1 0 At+1 (A A + B)

• If a bit in B is set to 1, that same position in A gets set to 1, otherwise that bit in A keeps its previous value

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SELECTIVE COMPLEMENTLogic Microoperations

• In a selective complement operation, the bit pattern in B is used to complement certain bits in A

1 1 0 0 At

1 0 1 0 B

0 1 1 0 At+1 (A A B)

• If a bit in B is set to 1, that same position in A gets complemented from its original value, otherwise it is unchanged

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SELECTIVE CLEARLogic Microoperations

• In a selective clear operation, the bit pattern in B is used to clear certain bits in A

1 1 0 0 At

1 0 1 0 B

0 1 0 0 At+1 (A A B’)

• If a bit in B is set to 1, that same position in A gets set to 0, otherwise it is unchanged

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MASK OPERATIONLogic Microoperations

• In a mask operation, the bit pattern in B is used to clear certain bits in A

1 1 0 0 At

1 0 1 0 B

1 0 0 0 At+1 (A A B)

• If a bit in B is set to 0, that same position in A gets set to 0, otherwise it is unchanged

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CLEAR OPERATIONLogic Microoperations

• In a clear operation, if the bits in the same position in A and B are the same, they are cleared in A, otherwise they are set in A

1 1 0 0 At

1 0 1 0 B

0 1 1 0 At+1 (A A B)

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INSERT OPERATIONLogic Microoperations

• An insert operation is used to introduce a specific bit pattern into A register, leaving the other bit positions unchanged

• This is done as– A mask operation to clear the desired bit positions, followed by

– An OR operation to introduce the new bits into the desired positions

– Example

» Suppose you wanted to introduce 1010 into the low order four bits of A:1101 1000 1011 0001 A (Original)

1101 1000 1011 1010 A (Desired)

» 1101 1000 1011 0001 A (Original)

1111 1111 1111 0000 Mask

1101 1000 1011 0000 A (Intermediate)

0000 0000 0000 1010 Added bits

1101 1000 1011 1010 A (Desired)

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LOGICAL SHIFTShift Microoperations

• In a logical shift the serial input to the shift is a 0.

• A right logical shift operation:

• A left logical shift operation:

• In a Register Transfer Language, the following notation is used– shl for a logical shift left

– shr for a logical shift right

– Examples:

» R2 shr R2

» R3 shl R3

0

0

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CIRCULAR SHIFTShift Microoperations

• In a circular shift the serial input is the bit that is shifted out of the other end of the register.

• A right circular shift operation:

• A left circular shift operation:

• In a RTL, the following notation is used– cil for a circular shift left– cir for a circular shift right– Examples:

» R2 cir R2» R3 cil R3

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Logical versus Arithmetic Shift

• A logical shift fills the newly created bit position with zero:

• An arithmetic shift fills the newly created bit position with a copy of the number’s sign bit:

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ARITHMETIC SHIFTShift Microoperations

• An left arithmetic shift operation must be checked for the overflow

0

VBefore the shift, if the leftmost twobits differ, the shift will result in anoverflow

• In a RTL, the following notation is used– ashl for an arithmetic shift left– ashr for an arithmetic shift right– Examples:

» R2 ashr R2» R3 ashl R3

signbit

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HARDWARE IMPLEMENTATION OF SHIFT MICROOPERATIONS

Shift Microoperations

S

01

H0MUX

S

01

H1MUX

S

01

H2MUX

S

01

H3MUX

Select0 for shift right (down) 1 for shift left (up)Serial

input (IR)

A0

A1

A2

A3

Serialinput (IL)

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ARITHMETIC LOGIC SHIFT UNIT

S3 S2 S1 S0 Cin Operation Function0 0 0 0 0 F = A Transfer A0 0 0 0 1 F = A + 1 Increment A0 0 0 1 0 F = A + B Addition0 0 0 1 1 F = A + B + 1 Add with carry0 0 1 0 0 F = A + B’ Subtract with borrow0 0 1 0 1 F = A + B’+ 1 Subtraction0 0 1 1 0 F = A - 1 Decrement A0 0 1 1 1 F = A TransferA0 1 0 0 X F = A B AND0 1 0 1 X F = A B OR0 1 1 0 X F = A B XOR0 1 1 1 X F = A’ Complement A1 0 X X X F = shr A Shift right A into F1 1 X X X F = shl A Shift left A into F

Shift Microoperations

ArithmeticCircuit

LogicCircuit

C

C 4 x 1MUX

Select

0123

F

S3S2S1S0

BA

i

A

D

A

E

shrshl

i+1 i

ii

i+1i-1

i

i

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HW 7

1. A Switch-tail ring counter (John counter) uses the complement of the serial output of a right shift register as its serial input. Starting from an initial state 0000, list the sequence of states after each shift until the register returns to 0000. (Q7-9a)

2. Use D-type flip flops and gates to design a counter with the following repeated binary sequence: 0, 1, 3, 2, 4, 6. (Q7-18)

Education

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